Paramedics became concerned after being called to Valley Manor Community Care Home in Castro Valley, where 14 patients were left behind and had to be evacuated Saturday.
A note on the door posted by the state Department of Social Services says the facility was to be closed Oct. 24. Paramedics found that most of the staff had left on Thursday, and only a skeleton crew remained.
Alameda County Sheriff's deputies are trying to find places for the abandoned residents.
Sheriff's Sgt. J.D. Nelson said some staff members stayed, including a cook, janitor and a single caretaker.
The Associated Press contributed to this report.